Bubble domain technology requires the deposition of iron garnet films of high perfection on single crystal substrates. These substrate crystals have to be of equal perfection as that required for the epilayer, and must be perfectly matched with their lattice constants to the film material and vice versa. In the course of technical development, wafers of gadolinium-gallium-garnet (GGG) material has proven to be very attractive. Efforts have concentrated on this material and it has emerged as the prime substrate for bubble domain memory work.
The standard technology used for the preparation of single crystals and wafers of GGG results in the generation of 80% by-products. These by-products are contaminated with a variety of non-transition and transition metal impurities introduced during fabrication. Since the presence of impurity ions in the crystal alter the lattice constants and render the substrate unusable for bubble domain material, the by-products could not be reintroduced into the production cycle. As this scrap consists of relatively expensive starting materials, the development of process technology to purify this material by a practical procedure has considerable economic value.
In U.S. Pat. No. 4,198,231 a method that involves digesting the scrap material, precipitating the gadolinium and electrolytically depositing the gallium is disclosed. Attempts at liquid-liquid extraction of the gadolinium gallium garnet residues dissolved in HCl and in HNO.sub.3 have also been attempted. However, the economics of processes requiring either liquid-liquid extractions of electrolytic deposition of the product associated with the recovery of gallium have rendered such methods impractical on an industrial scale. In the pending U.S. patent application No. 334,719 filed on Dec. 28, 1981, now U.S. Pat. No. 4,375,453, a process is described involving the digestion of the scrap, precipitation of the gadolinium and gallium as sulfate and oxalate salts and pyrolysis to convert them to the oxides. While this latter method is more cost effective, the present application provides an alternative method which is also efficient and economical and in various respects is more applicable to existing production facilities.